I. Field of the Invention
The present invention relates to communication. More particularly, the present invention relates to a novel and improved method and apparatus for performing idle mode reacquisition and handoff in an asynchronous wireless communication system.
II. Description of the Related Art
In a typical wireless communication system, the remote terminals are only sporadically active. For example, a cellular telephone remains idle for significant periods of time when no call is in progress. However, to ensure that any message directed to a remote unit is received, the remote unit periodically monitors the communication channel, even while it is idle. Messages transmitted by the base stations may include those for alerting the remote terminal to the presence of an incoming call and those for updating system parameters in the remote terminal.
During idle mode, the remote terminal continues to consume power to sustain the circuitry necessary to monitor the signals from the base stations. Many remote terminals (e.g., cellular telephones) are portable and powered by an internal battery. The power consumption by the remote terminal in the idle mode decreases the available battery resources, which shortens the xe2x80x9cstandbyxe2x80x9d time between battery recharges and the xe2x80x9ctalkxe2x80x9d time when a call is placed or received. Therefore, it is highly desirable to minimize power consumption in the remote terminal in the idle state and thereby increase battery life.
In one technique for reducing power consumption in a remote terminal operating in the idle mode, the remote terminal periodically enters an xe2x80x9cactivexe2x80x9d state during which it can receive messages on a paging channel from the base stations with which it has previously established communication (i.e., the base stations in the active set). The paging channel may be divided into numbered xe2x80x9cframesxe2x80x9d (e.g., frames 0 through 1023) and the remote terminal may be assigned one or more frames by the base stations. (A frame may also be referred to as a xe2x80x9cslotxe2x80x9d or a xe2x80x9ctime-framexe2x80x9d in some other CDMA systems.) Thereafter, the remote terminal wakes up from an xe2x80x9cinactivexe2x80x9d state prior to its assigned frame, monitors the paging channels for messages, and reverts to the inactive state if additional communication is not required. In the time period between successive active states, the remote terminal is in the inactive state and the base stations do not send any messages to the remote terminal. In the inactive state, as much circuitry as possible is powered down to conserve power.
Before entering the inactive state (or power saving mode), a remote terminal communicates its presence to a xe2x80x9cpreferredxe2x80x9d base station, which is also often referred to as a xe2x80x9cserving cellxe2x80x9d. Typically, the preferred base station is the base station that has the strongest pilot signal as measured by the remote unit. During its active state, the remote unit measures the pilot signal strength of the preferred base station as well as the pilot strengths of neighboring base stations. If the remote unit relocates from the coverage area of the preferred base station to a neighboring base station""s coverage area, the remote unit needs to xe2x80x9chandoffxe2x80x9d communication to the neighboring base station. When a handoff occurs, the neighboring base station is assigned as the new preferred base station.
The preferred base station""s pilot signal is typically stronger than any of the measured neighbor pilot signals when the remote unit enters its inactive state. Therefore, when the remote unit enters the next active state, it monitors the preferred base station. However, while the remote unit is in its inactive state, it may have relocated from the coverage of the preferred base station into the coverage of a neighboring base station. When the remote unit is in its inactive state, it does not monitor the signal strengths of the preferred and neighboring base stations. Moreover, even if the neighboring base station""s signal has increased to a signal strength sufficiently greater than that of the preferred base station for a handoff to occur, the remote unit does not perform a handoff while in the inactive state.
Thus, when the remote unit returns to the active state, it may not be monitoring the xe2x80x9coptimumxe2x80x9d base station. In fact, the signal strength from the preferred base station may be so low that the remote unit is unable to properly decode the information transmitted from this base station, and may miss important pages intended for the remote terminal. For a remote terminal to monitor the optimum base station, the remote terminal measures the preferred base station and some candidate neighbor base stations, and handoff to a strong neighboring base station if necessary before the remote terminal""s assigned page time.
For a synchronous communication system, such as an IS-95 CDMA system, the timing of all base stations in the system is aligned. A remote terminal can thus be assigned the same time-aligned frame by the base stations. From the remote terminal""s perspective, its assigned page time is the same for the preferred base station as for neighboring base stations in a synchronous communication system. By knowing the timing of the preferred base station, the remote terminal is able to determine its assigned page time on the preferred as well as neighboring base stations. By synchronizing its timing to the preferred base station""s timing, the remote terminal automatically synchronizes its timing to that of the neighboring base stations. Therefore, if the remote terminal can handoff to a neighboring base station before its assigned page time on the old preferred base station, the remote terminal can decode its assigned page information on the neighbor base station on time.
Some newer generation wireless communication systems are not synchronous, and the timing (and thus, frames) of the base stations is not aligned. As a result of the asynchronous timing among base stations, the assigned page time, from a remote terminal""s perspective, may be different from base station to base station. By knowing the timing and the assigned paging time of the preferred base station, the remote terminal typically cannot determine the assigned paging time on a neighboring base station. If the remote terminal moves to the neighboring base station""s coverage area, the remote unit needs to synchronize its timing to this neighboring base station""s timing in order to decode information transmitted from this base station. Therefore, if a remote terminal tries to wake up to decode paging information on the preferred base station but ends up handing off to a neighboring base station due to change in coverage area, the remote terminal may completely miss the assigned paging time of the neighboring base station, and may thus miss important paging information.
Therefore, there is a need in the art for a method and apparatus for performing idle mode reacquisition and handoff in asynchronous wireless communication systems.
The invention provides various techniques to efficiently process paging channels in an asynchronous wireless communication system. In accordance with one aspect of the invention, if the base stations are not synchronized with each other and a remote terminal""s assigned paging time is different from base station to base station (from the remote terminal""s perspective), the remote terminal wakes up based on the earliest base station (from the remote terminal""s point of view) in a xe2x80x9creacquisition searchxe2x80x9d list. This list includes the preferred base station as well as neighboring base stations to which the remote terminal may be handed off at a subsequent wake-up interval. This allows the remote terminal to perform reacquisition searches and reacquisition handoff and avoid missing pages.
In accordance with another aspect of the invention, the criteria to select base stations for the reacquisition search list may be based on the base station timing as well as the measured signal strength of the base stations, both of which may be made relative to that of the preferred base station. In an embodiment, if, from the remote terminal""s point of view, a neighboring base station""s timing is in close proximity of the preferred base station""s timing, and if the neighboring base station""s signal is measured strong enough before entering the inactive state, then this neighboring base station is added to the reacquisition search list, together with the preferred base station. The next scheduled wake-up time (or active state) is then based on the earliest of the preferred and neighboring base stations in the list. In this manner, the preferred and neighboring base stations will be searched and evaluated, and the paging information from the base station with strongest signal is decoded, at the next active state.
Otherwise, if a neighboring base station""s timing is outside the proximity of the preferred base station""s timing, and if the neighboring base station""s signal is measured sufficiently stronger than the preferred base station before entering the inactive state, then this neighboring base station is added to the reacquisition search list, together with the preferred base station. However, the next scheduled wake-up time (or active state) is based on the neighboring base station""s paging channel timing. In this case, this neighboring base station and the preferred base station will be searched and evaluated at the next active state. If the neighboring base station is still sufficiently stronger than the preferred base station, then it will become the new preferred base station and the paging information from this base station will be decoded. However, if the neighboring base station is no longer sufficiently stronger than the preferred base station, the remote terminal will not decode any paging information. Instead, the remote terminal enters the inactive state immediately and prepares for reacquiring and decoding the preferred base station at the next active state.
By dividing the neighboring base stations into different categories according to their timing relation to the preferred base station, the amount of time the remote terminal needs to be in the active state may be reduced, which may translate to improvement in the remote terminal""s stand-by time, without missing important paging information.
In accordance with yet another aspect of the invention, the remote terminal may wake up multiple times in a particular paging cycle (i.e., a DRX cycle) if the preferred base station and strong neighboring base stations (handoff candidates) are outside the timing proximity of each other. A neighboring base station may be identified as a handoff candidate if it meets a set of criteria, which may be based on the base station energy measurement and a reselection timer value, as described in further detail below.
Various aspects, embodiments, and features of the invention are described in further detail below.